Code division multiple access (CDMA) modulation techniques are used in various communications fields to allow multiple access (i.e., multiple data signals to be transmitted on a single channel). According to CDMA, each data signal is modulated with a pseudo-random code having a wider bandwidth than the data signal. The result is a spread-spectrum user signal or code. Different data signals to be transmitted on the same channel may utilize different pseudo-random codes. A receiver of a particular data signal may either have or may derive the pseudo-random code with which its respective data signal was modulated. The receiver may utilize the appropriate pseudo-random code to demodulate the user signal or code and derive the original data signal. User signals or codes modulated by pseudo-random codes not known to the receiver will appear as noise.
It is often desirable to amplify a CDMA signal composed of many individual user signals or codes (e.g., to transmit the signal). For example, a global positioning system (GPS) satellite may transmit a combination of multiple GPS signals. Also, for example, a mobile telephone base station may transmit a combination of multiple CDMA signals corresponding to different conversations or other data transactions. In many applications, highly linearized power amplifiers (LPA's) may be used to amplify signals comprising a combination of codes. LPA's generate an output signal with minimal distortion, however, suffer from poor power efficiency. When a higher power efficiency is required, high-power nonlinear power amplifiers (HPA's) may be used. HPA's, however, generate a large number of distortion terms that are present, along with the input signal, at the amplifier output. In addition, there is some loss in the signal power as a component of the amplifier output power is used up by the distortion terms. It is possible to minimize the distortion terms and associated output power loss by configuring the HPA with a larger output power back-off. This, however, also reduces DC-RF power conversion efficiency.
It is known to reduce HPA distortion by pre-combining CDMA codes prior to amplification according to a majority logic scheme. A majority logic combiner counts the number of codes at its input that have a value of +1 and −1 during any chip period. The output is set to which ever value (e.g., +1 or −1) occurs in a majority of the signals. Where M is the number of codes to be combined, the output of a majority logic combiner at any time k may be equal to +1 if the number of codes that take the value +1 is greater than or equal to (M+1)/2 and is equal to −1 otherwise. For example, Table 1 below shows a chart illustrating the output (v0) of a majority logic combiner operating on three input codes (c1, c2, c3):
TABLE 1Majority Logic Combiner Outputc1−1−1−1−1+1+1+1+1c2−1−1+1+1−1−1+1+1c3−1+1−1+1−1+1−1+1v0−1−1−1+1−1+1+1+1The output (v0) may be provided to an HPA amplifier. Note that in a majority logic combiner, M must be odd to avoid the occurrence of a tie.
Majority logic combining may allow HPA amplifiers to operate more effectively, however, the majority logic combining process itself may introduce distortion terms, which can be amplified by the HPA, leading to signal degradation.